This invention is directed to an improvement in the art of making hot pressed silicon nitride from silicon or silicon nitride powder as a starting material. When starting with silicon powder, such powder is first subjected to a reactive nitriding gas to form a mixed phase polycrystalline silicon nitride rigid body, the body being then hot pressed. Within this art there are several ways to agglomerate the powder materials: (a) by cold compacting or forming into a powder preform as an early stage in the process before or without nitriding (see U.S. Pat. No. 3,839,540), (b) nitriding without compaction (see U.S. patent application Ser. No. 448,889, commonly assigned to the assignee herein), and (c) by hot pressing, without prior compaction or nitriding, as the last stage in the process with Si.sub.3 N.sub.4.
The preform or resulting cake, using any of the above agglomeration techniques, presents a severe height to width ratio requiring an unusually long hot pressing stroke to obtain full densification of the material. For example, the cake or preform can have a density of 0.9-2.5 gm/cm.sup.3 and must be converted to a density of 3.25 gm/cm.sup.3 or above. A long pressing stroke, particularly under the high temperature and pressing conditions associated with hot pressing, induces significant side wall drag, a friction impediment at the interface between the sides of the cake or billet and the interior wall of the pressing cavity. The side wall drag, in conjunction with the thermal and pressure gradients within the material, produces "material transport," leading to a dishing effect of the resulting pressed product which can be quite pronounced.
What is needed is a method by which relatively low density cakes or preforms can be hot pressed without the above recited disadvantages.